Finite Lifetime Fragment Model 4 for Striae Formation in the Dust Tails of Comets (FLM 4) Acceleration by Lorenz-force

TL;DR

The paper tackles the longstanding question of how striae form in comet dust tails by proposing FLM4, a mechanism in which refractory particles near the nucleus experience Lorentz-force acceleration that varies as their radii decay. By coupling a near-nucleus acceleration cylinder with a size-dependent decay process, FLM4 yields a time-varying acceleration ratio $\beta=\beta_f+\beta_i+\beta_g$ that naturally produces the observed striae morphologies and brightness, while reducing the parameter set to five independent values plus constants $C_L$ and $R$. Compared to FLM3, FLM4 achieves comparable explanatory power with fewer degrees of freedom and without requiring composite particles, offering a more natural physical basis for striae formation. The model successfully reproduces six striae across three comets (Hale-Bopp, West, Seki-Lines) and provides a framework for predicting striae shapes and luminosities under varying solar-wind and illumination conditions, advancing our understanding of dust dynamics in cometary environments.

Abstract

The striations in the dust tails of comets are referred to as striae, and their origin has long been a mystery. We introduce a new dynamic model to describe the forms of the striae observed in comets Hale-Bopp (C/1995 O1), West (C/1975 V1), and Seki-Lines (C/1962 C1). Charged particles made of refractory materials, with radii less than 0.5micrometer, are expelled from the comet's nucleus and accelerated by Lorentz forces near the nucleus. These particles decay many times to form striae, which have a lifespan of less than about 100 days at a distance of 1 astronomical unit from the sun. Over time, they continue to decay and eventually disappear from view. The following dynamic model explains these material science processes. Particles expelled from the comet's nucleus are subjected to three forces: solar gravity, solar radiation pressure, and Lorentz forces near the nucleus. As these particles decrease in size, the Lorentz forces and radiation pressure cause fluctuations, increasing and decreasing to form striae. This model, which is less of a dynamic approximation than previous theories (FLM3), explains the structure of the striae, enables predictions of their luminosity, and clarifies their origin.

Figures (11)

• Figure 1: Comet West (C/1975 V1) on March 6.810, 1976. Many striae can be seen in the dust tails. The characters indicate the names of striae. Observer: the author (Nishioka, 1998); Lens: f.l. 55mm Fno. 2.8; exposure: 8 min; emulsion: Kodak 103aE, without filter.
• Figure 2: Schematic diagram of the magnetic field $B$, current $J$, Lorentz force $L$, ion tail, and dust tail near the comet nucleus. The line connecting the Sun and the comet nucleus is called the central line. The dashed line indicates the ion tail, while the curved solid line indicates the dust tail. Magnetic field lines parallel ($\otimes$) and antiparallel ($\odot$) to the central line create a U-shape. Perpendicular to these, the magnetic field $B$ and the flow of ions and charged particles (current $J$) generate the Lorentz force $L$. The acceleration cylinder has the central line as its center and a radius of CL, with the nucleus at the center on the Sun-facing side. Charged particles experience the acceleration $\beta_i$ only when inside the acceleration cylinder. This figure is a modification of Saito (1989).
• Figure 3: Schematic Diagram of the Time Variation of the Ratio $\beta$ of the Acceleration Experienced by Charged Particles to Gravitational Acceleration. (Time- Acceleration function) The horizontal axis represents time, and the vertical axis represents the ratio of the acceleration experienced by charged particles to gravitational acceleration. The dotted line represents $\beta_f$, the dashed line represents $\beta_i$, and the solid line represents the time variation of $\beta$. ti is the time when the charged particles exit the acceleration cylinder. Particles emitted at time te experience an increase in $\beta_i$ as they disintegrate, reaching zero at ti. After ti, $\beta_i$ remains zero. $\beta_f$ increases after te as the particles disintegrate, peaks, and then decreases. After ti, $\beta$ is equal to $\beta_f$. Therefore, $\beta$ increases after te, decreases at ti, and then rises and falls again. This repeated rise and fall of $\beta$ is the fundamental mechanism for the formation of striae. Small arrows along the graph indicate the time evolution of $\beta$. trajectory of the particles ejected from the nucleus with 0 initial velocity lies on the orbital plane of the comet. The time variation of $\beta$ affects the perihelion distance, the eccentricity and the argument of perihelion of the orbital element of the particle. The mechanism of fragmentation of the particle is out of scope of this study.
• Figure 4: A diagram illustrating the fragmentation process by which particles ejected from a comet's nucleus form a stria. The horizontal axes depict time (extending to the right) and particle size (extending to the left). Refractory particles ($\bigcirc$) are emitted from the comet's nucleus at time te. These particles break down over time, continuing to disintegrate until they become invisible. According to FLM4, all particles, except those that become invisible, are observed as striae during this process.
• Figure 5: Table 1. The asterisk marks denote the independent parameters of FLM4. Units are shown in parentheses. CL and R are constants specific to FLM4 and take nearly identical values for all striae. For comets Hale–Bopp (C/1995 O1) stria J and West (C/1975 V1) stria VI, the same parameters explain observations over a span of two days.